1. Field of the Invention
The present invention relates to a photomask used for forming a fine pattern in fabrication of a semiconductor integrated circuit device and a pattern formation method using the photomask.
2. Description of Related Art
Recently, there are increasing demands for refinement of a circuit pattern due to a high degree of integration of a large scale integrated circuit device (hereinafter referred to as an LSI) realized by using a semiconductor. As a result, great importance is placed on refinement of a line pattern forming the circuit or a contact hole pattern for connecting multiple wiring layers through insulating layers. For the formation of the line pattern, a technique of simultaneously forming isolated patterns and densely arranged patterns has been required. For achieving large depth of focus in the formation of the densely arranged line patterns, it is essential to perform oblique-incidence exposure (off-axis exposure). However, the oblique-incidence exposure considerably deteriorates contrast and the depth of focus of the isolated line patterns. The contrast and the depth of focus are deteriorated more significantly when a half-tone phase-shifting mask, which is widely used for improvement in resolution, is used. On the other hand, when a light source of low coherence suitable for the formation of isolated fine line patterns is used, the formation of the dense patterns becomes difficult.
As described above, the optimum lighting condition for the isolated fine line patterns is contradictory to the optimum lighting condition for the densely arranged line patterns. Therefore, in order to form the densely arranged fine patterns and the isolated patterns at the same time, tradeoff is made on the effect of a vertical incident light component and the effect of an oblique incident light component emitted from a light source. As a result, a light source having a medium degree of coherence (about 0.5 to 0.6) is used. In this case, however, the effects of the vertical incident light component and the oblique incident light components cancel each other. This makes it difficult to achieve the simultaneous refinement of the dense patterns and the isolated patterns to enhance the degree of integration of the semiconductor device. One of the most effective solutions to this problem is use of an enhancer mask (see Patent Document 1).
Hereinafter, the principle of pattern formation using the enhancer mask according to Patent Document 1 is explained with reference to FIGS. 12A to 12D. FIGS. 12A and 12B show an exemplified plane structure and an exemplified cross-sectional structure of the enhancer mask. As shown in FIGS. 12A and 12B, the enhancer mask includes a transparent substrate 11 having a transparent property against exposing light, on which provided are a semi-light-shielding portion 12 having a predetermined transmittance to the exposing light, an auxiliary pattern 13 surrounded by the semi-light-shielding portion 12 and an opening 14 in which the semi-light-shielding portion 12 and the auxiliary pattern 13 are not formed. The semi-light-shielding portion 12 and the opening 14 allow the exposing light to pass through in an identical phase. The auxiliary pattern 13 allows the exposing light to pass through in an opposite phase with respect to the semi-light-shielding portion 12 and the opening 14, but is not transferred on a wafer by exposure.
FIG. 12C is a diagram for showing a light intensity profile obtained by using the enhancer mask of FIGS. 12A and 12B, and FIG. 12D is a schematic diagram for showing the plane structure of a pattern to be transferred onto a wafer by the enhancer mask of FIGS. 12A and 12B. As shown in FIG. 12C, in the photomask shown in FIGS. 12A and 12B, light passing through the auxiliary pattern 13 surrounded by the semi-light-shielding portion 12 cancels a portion of light passing through the opening 14 and coming around the back of the semi-light-shielding portion 12. Therefore, if the intensity of light passing through the auxiliary pattern 13 is adjusted so that the light coming around the back of the semi-light-shielding portion 12 is canceled, a dark portion in which the light intensity is reduced to approximately 0 is formed in the light intensity distribution.
The light passing through the auxiliary pattern 13 pronouncedly cancels the light coming around the back of the semi-light-shielding portion 12, while it slightly cancels the light passing near the edge of the semi-light-shielding portion 12. As a result, the light intensity profile of the light passing through the photomask of FIGS. 12A and 12B becomes steep in part thereof from the vicinity of the semi-light-shielding portion 12 to the center of the auxiliary pattern 13. Thus, the light passing through the photomask of FIGS. 12A and 12B, i.e., the enhancer mask, shows a sharp light intensity profile. Therefore, as shown in FIG. 12D, a high contrast image is formed.
As described above, with the provision of the auxiliary pattern 13 in the semi-light-shielding portion 12 having a light-shielding property, a considerably dark portion corresponding to a region (auxiliary pattern 13) surrounded by the outline of the semi-light-shielding portion 12 is formed in a light intensity image formed by using the photomask of FIGS. 12A and 12B. This makes it possible to provide light intensity distribution with enhanced contrast between the light intensity of the center of the auxiliary pattern 13 and the light intensity around the semi-light-shielding portion 12. Thus, the enhancer mask is highly effective for the simultaneous formation of the fine isolated line pattern (e.g., a pattern having a width smaller than (0.5×λ/NA) (λ: a wavelength of the exposing light, NA: numerical aperture)) and the densely arranged line patterns.    [Patent Document 1] Publication of Japanese Patent Application No. 2003-322949